Avoid temporary stack slot in drop codegen

cc #322

src/abi/mod.rs

@@ -700,18 +700,13 @@ pub(crate) fn codegen_drop<'tcx>(
             _ => {
                 assert!(!matches!(drop_fn.def, InstanceDef::Virtual(_, _)));
 
-                let arg_place = CPlace::new_stack_slot(
-                    fx,
-                    fx.layout_of(fx.tcx.mk_ref(
-                        &ty::RegionKind::ReErased,
-                        TypeAndMut {
-                            ty,
-                            mutbl: crate::rustc_hir::Mutability::Mut,
-                        },
-                    )),
-                );
-                drop_place.write_place_ref(fx, arg_place);
-                let arg_value = arg_place.to_cvalue(fx);
+                let arg_value = drop_place.place_ref(fx, fx.layout_of(fx.tcx.mk_ref(
+                    &ty::RegionKind::ReErased,
+                    TypeAndMut {
+                        ty,
+                        mutbl: crate::rustc_hir::Mutability::Mut,
+                    },
+                )));
                 let arg_value = adjust_arg_for_abi(fx, arg_value);
 
                 let mut call_args: Vec<Value> = arg_value.into_iter().collect::<Vec<_>>();

src/base.rs

@@ -405,7 +405,8 @@ fn trans_stmt<'tcx>(
                 }
                 Rvalue::Ref(_, _, place) | Rvalue::AddressOf(_, place) => {
                     let place = trans_place(fx, *place);
-                    place.write_place_ref(fx, lval);
+                    let ref_ = place.place_ref(fx, lval.layout());
+                    lval.write_cvalue(fx, ref_);
                 }
                 Rvalue::ThreadLocalRef(def_id) => {
                     let val = crate::constant::codegen_tls_ref(fx, *def_id, lval.layout());

src/value_and_place.rs

@@ -681,18 +681,20 @@ pub(crate) fn place_deref(self, fx: &mut FunctionCx<'_, 'tcx, impl Backend>) ->
         }
     }
 
-    pub(crate) fn write_place_ref(self, fx: &mut FunctionCx<'_, 'tcx, impl Backend>, dest: CPlace<'tcx>) {
+    pub(crate) fn place_ref(
+        self,
+        fx: &mut FunctionCx<'_, 'tcx, impl Backend>,
+        layout: TyAndLayout<'tcx>,
+    ) -> CValue<'tcx> {
         if has_ptr_meta(fx.tcx, self.layout().ty) {
             let (ptr, extra) = self.to_ptr_maybe_unsized();
-            let ptr = CValue::by_val_pair(
+            CValue::by_val_pair(
                 ptr.get_addr(fx),
                 extra.expect("unsized type without metadata"),
-                dest.layout(),
-            );
-            dest.write_cvalue(fx, ptr);
+                layout,
+            )
         } else {
-            let ptr = CValue::by_val(self.to_ptr().get_addr(fx), dest.layout());
-            dest.write_cvalue(fx, ptr);
+            CValue::by_val(self.to_ptr().get_addr(fx), layout)
         }
     }